Single longitudinal mode, with a fixed or narrowly tunable wavelength, semiconductor lasers are typically deployed in modulated transmitter modules for wavelength division multiplexed (WDM) optical data communication applications. Their semiconductor gain media can be directly modulated or the lasers can be deployed with separate external modulators based on Pockel's cells, directional couplers, Fabry-Perot etalons, Mach-Zehnder interferometers, or electroabsorptive chips.
A common module configuration includes a 1.3–1.6 micrometer (μm) distributed feedback Bragg (DFB) laser chip, a backfacet photodetector, an electroabsorptive modulator, a wavelength locker and associated electronics, a thermistor, an isolator, and an thermoelectric cooler. These devices are applicable to terminal equipment in WDM applications from metro to extended-reach systems, operating at 2.5 gigabits per second (Gbps) and higher.